The present invention involves chemical optimization to remove metallic alloys or films from a chemically compatible substrate. The use of metal alloys and substrates in the making of mechanical and electrical systems has become commonplace in many factories. While the use of and combination of metal alloys and metals in various processes is frequently desirable, it is also frequently necessary to remove or separate one metal from the other.
For example, as is known to those skilled in the art of manufacturing semiconductors, Aluminum Alloys and Titanium films are sputtered onto a semiconductor substrate. The Aluminum Alloy is used to form electrical connections on the surface of the substrate. As is known to those skilled in the art, the machines commonly used to apply such Aluminum Alloys to the substrate are called sputterors or evaporator systems and fall under the label of physical vapor deposition devices. Such evaporator systems deposit patterned Aluminum Alloy wiring on the substrate, as is known to those skilled in the art. The Aluminum Alloy is commonly composed of Aluminum and Silicon, but can also include Aluminum and Titanium, or Aluminum alloyed with other materials. The substrate is commonly Silicon, but can be Gallium Arsenide or other semiconductor material.
Generally described, the evaporator system is utilized to deposit the Aluminum Alloy film on the surface of semiconductor wafers to form wire connections. Although it is not necessary to understanding the present invention, a brief summary of the operation of the evaporator is supplied as background. Evaporation or sputtering is accomplished using a liquefied metallic material (commonly an Aluminum Alloy). A target surface is bombarded with energetic ions that dislodge target atoms from the target surface. Under proper conditions the "sputtered" atoms are transported to the surface of semiconductor wafers where they deposit to form a thin film. It is during this operation that the Aluminum Alloy is also deposited on the Stainless Steel substrate of the evaporator system.
During patterning the Aluminum Alloy thus adheres to the Stainless Steel parts of the evaporator system. In other words, an unwanted layer or build-up of the Aluminum Alloy will be deposited on the Stainless Steel evaporator as a by-product of using the evaporator system to deposit wiring on the semiconductor substrate. To ensure continued usability of the evaporator system, the Aluminum Alloy film must be removed from the evaporator system.
The common manner in the industry for removing the Aluminum Alloy involves the use of acid baths to clean the Aluminum Alloys from the Stainless Steel. Commonly used solutions include a mixture of Hydrogen Peroxide and Phosphoric Acid. However, such a mixture applied to the Aluminum Alloy deposit produces a brown Silicon residue that remains on the evaporator system. The Silicon residue will not be removed by the Hydrogen Peroxide and Phosphoric Acid bath. This residue adheres to the evaporator system and contaminates future operation of the evaporator. Further, the acids often etches the Stainless Steel as well as the Aluminum Alloy, thereby permanently damaging the evaporator system to inhibit or preclude further use of the system.
There are many other examples of removing alloy films from a chemically compatible substrate. Such a film might be a copper alloy that is deposited on a glass surface. Removing or patterning the alloy without damaging the substrate is difficult, but necessary in industrial applications.
Processes known to those skilled in the art for patterning or removing alloys from chemically compatible substrates typically entail multiple step exposures to a variety of acid baths, washes and other solutions to clean specific metals. For example, in the semiconductor industry it is known to those skilled in the art that Buffered Oxide Etch is a solution that etches Titanium, whereas Nitric acid does not etch Titanium. As is known to those skilled in the art, Buffered Oxide Etch is a mixture of Ammonium Fluoride and Hydrofluoric Acid. Thus, to etch or strip an Aluminum-Titanium Alloy, Buffered Oxide Etch and another solution must be used to strip both materials. Further chemical and :mechanical steps are generally required to remove the alloy from the substrate.
However, the proper composition or mixture to etch various Aluminum Alloys and Titanium films from substrates without damaging the underlying substrate was not known. Further, the user does not know what is the proper solution to most quickly remove the alloy from the substrate.
Thus, it would be desirable to have an etching or stripping solution that optimally removes various Aluminum and Copper Alloys from chemically compatible substrates with limited etching of the substrate.